The demand for upgrading the spectral efficiency of optical networks is ever increasing as internet traffic continues to increase almost exponentially. The move from 10 G to 40 G has been proposed for some time, but has not yet become the norm in optical communications. There are economic reasons behind this, such as the internet bubble in the early 2000s. But the main reason is still that the technology has not been good enough. The presently widely deployed 10 G system employs the NRZ-OOK amplitude modulation format and the direct detection scheme and the DWDM system (50 G channel spacing).
Erbium Doped Fibre Amplifiers (EDFAs) and chromatic dispersion compensation modules are used to compensate loss and dispersion along the optical link in a periodic manner. Directly upgrading such a system to 40 G by simply using a 40 G NRZ-OOK modulation format and the direct detection scheme and the same optical link will not be possible. There are two main reasons to impede this. The first one is channel spacing. The 40 G NRZ-OOK signal cannot be put on channels with 50 G spacing. The other reason is that 40 G NRZ-OOK signal has much less tolerance to chromatic dispersion and polarization mode dispersion than 10 G so it can not directly propagate on present optical links.
Of more importance, there is a lot of work carried out around the world to try to change the NRZ-OOK modulation format and the direct detection scheme to other more advanced phase modulation formats and detection schemes. The main direction of innovation is to reduce the symbol rate by increasing the number of bits of information carried by each symbol. The most sophisticated and also the most successful is the Dual-Polarization Quadrature-Phase Shift Keying (DP-QPSK) format combined with the digital coherent detection scheme. For this scheme each symbol carries four bits of information, which means that a 40 G signal can be achieved with a 10 G symbol rate. It is easily understandable that this 40 G signal would comply with all the requirements of presently available 10 G systems but a coherent receiver is now essential for operation. Due to the digital coherent detection scheme, the signal's amplitude and phase are detected and digitized so all the dispersions and non-linear impairments that the signal has suffered from the optical link can be compensated at the end using sophisticated digital filters. So this 40 G system, if implemented, is actually a huge improvement over the original 10 G NRZ-OOK system.
But these benefits will not come along without costs and more complicated network architectures. The DP-QPSK modulation formats will need more complex transmitters. The digital coherent detection scheme needs a full coherent receiver (dual polarization and dual quadrature). Currently commercially available coherent receivers are based on miniaturized free-space optics. Many optical elements like mirrors, splitters, and photodetectors are needed to be positioned very precisely. This makes this type of detector less competitive in terms of volume, cost, yield and reliability. There are also schemes to use hybrid integration that is to use for example Silica optical circuits to form the polarization beam splitters and optical hybrids and then couple these passive optical circuits to photodetectors. This type of hybrid integration still needs precise alignment between passive waveguides and photodetectors. Integrating all of these optical components onto a single chip would be strongly desired. Considering the material for photon detection at the optical wavelength used for optical communications is mainly Ge on Silicon or InGaAs on InP, the natural integration platform would be Silicon based or InP based. However realizing the functions of polarization beam splitters and optical hybrids on these two material platforms is problematic, especially for polarization beam splitters.
A known coherent receiver, as shown in FIG. 1, requires two polarization beam splitters (PBS), two 90 degree optical hybrids, and 8 high speed photodiodes (4 balanced photodiodes (BPD)). At the moment all commercially available coherent receivers are based on miniaturized free-space optics. In the long run and for large scale manufacture an integration scheme to realize such a complex receiver is essential. In the 2009 Optical Fibre Communication (OFC) conference Alcatel-Lucent demonstrated such a receiver integrated on silicon. However, the relatively mature approaches are still based on miniaturized free-space optics. As such, these are expensive solutions due to the requirement to physically integrate all the various components.
The available commercial products include the single polarization 90 degree optical hybrid, dual polarization 90 degree optical hybrid from Kylia, http://kylia.com/compa.html and a single polarization 90 degree optical hybrid from Optoplex, http://www.optoplex.com/Index.htm. However a problem with these solutions is that they are still optical hybrids which have to be combined with balanced photodiodes and fibre connections to form a functioning coherent receiver. This is very challenging and therefore costly because the fibre connection length must be controlled to high precision. To avoid the difficulty for the accurate fibre length control, the balanced photodiodes can be installed together with the optical hybrids. This has been done by the company CeLight who uses a LiNbO3 based optical hybrid, http://www.celight.com/.
At the 2011 Optical Fibre Communication (OFC) conference there were two reports on integrated coherent receivers one from Alcatel-Lucent who did the integration on silicon (as mentioned above). The second report is from u2t which is a German company specialized on making high speed photodiodes and balanced photodiodes. They have managed to integrate one 90 degree optical hybrid and four photodiodes (two balanced photodiodes) on a single chip. In their integration the 90 degree optical circuit is realized by a 4 by 4 MMI (Multi-Mode Intererometer) beam splitter which is simple but has very tight fabrication tolerance. There will be difficulties to further integrate the polarization beam splitter because the waveguides are ion-doped so they are purely passive.
It is therefore an object of the invention to provide a coherent receiver to overcome the above mentioned problems.